The present invention relates generally to force measurement, and more particularly to a force transducer assembly which measures both tensile and compressive forces applied via a force transmission device and converts applied forces into pressures in two pressure sensing capsules.
It is desirable to record whether a force transmission device, such as a connecting rod in a rudder control system of an airplane for example, is being subject to compressive or tensile loads, and to determine the magnitude of those applied loads. For force transmission devices of this type, such as linkages attached to a control wheel, control column, elevator control or rudder control pedal of an airplane for example, it is known to be desirable to measure what type and with what magnitude a force is being applied to it. In this way one can determine the direction a rudder or other device was moved in and the magnitude of a force applied. It is desirable to measure the forces being applied to these, as well as other types of transmission devices, so that appropriate data may be stored for later retrieval, e.g. in an airplane flight recorder. Such devices are frequently called xe2x80x9cload cellsxe2x80x9d and most such devices usually transmit tensile or compressive applied forces to an internal load column which in turn is stressed in tension or compression. The resulting stress in the column gives rise to a strain in the column which is measured by affixing xe2x80x9cstrain gagesxe2x80x9d to the column and by connecting the xe2x80x9cstrain gagesxe2x80x9d into a Wheatstone Bridge which provides an output which is proportional to the strain and hence the applied load. Such a device is inherently costly because of the required close dimensional tolerances of the column and the exactness of the placement of the individual strain gages. In addition, it is also very labor intensive because each individual strain gage must be separately applied to the column and only then can the four strain gages be connected into a Wheatstone Bridge. Moreover, any difference in temperature across or along the column can result in error signals, since each strain gage can change its resistance because of a change in temperature.
Accordingly, it is an object of the present invention to provide a different way to sense the application of tensile and compressive forces by separately converting them into a pressure for each force. By using two pressure capsules, only one of which undergoes a pressure increase during a tensile load while the other capsule undergoes a pressure increase only during a compressive load, and by using a half-bridge sensor configuration from each sensor, an output proportional to the load can be obtained.
A force sensor assembly for measuring compressive and tensile loads applied to a force transmission device including first and second ends, the assembly including: a housing coupled to the first end of the force transmission device and defining an interior; a sleeve assembly coupled within the interior of the housing and to the second end of the force transmission device; and, a sensor device secured within the interior of the housing between a portion of the sleeve assembly and a portion of the housing, the sensor device including first and second sensor capsule, each capsule including an isolation diaphragm at least partially defining an oil-filled cavity and a piezoresistive sensor positioned so as to be effected by a change in pressure in the oil-filled cavity, whereby when a first force is applied to the apparatus via the force transmission device, one of the capsules is subjected to a pressure deflects and when a second force is applied, the other of the capsules is subjected to a pressure; each capsule containing a halfbridge and the two half-bridges interconnected to form a full bridge. Thus, the resulting full bridge will indicate either a tensile or a compressive force.